It is well known that ‘cable’ television has largely supplanted broadcast television as the preferred real-time video signal distribution method of choice in the modern home. Compared to broadcast technology, cable distribution offers superior protection against signal degradation and greater available bandwidth. Consumers have expressed a clear preference for programming quantity and variety, and the demand for large channel menus seems likely to persist.
Video signals arriving at a multiple dwelling unit building, either by feeder cable or satellite dish or some other source, are advantageously distributed by a local network of coaxial cable terminating in each dwelling unit. These cables provide good shielding, both from outside sources of interference and from interference from the cable signals themselves with broadcast reception. Concurrently, coaxial cables provide low loss characteristics. This technically sound distribution scheme is not without disadvantages, however. Wiring a building for cable entails a considerable labor expense, and, depending on the care of the installers, a more or less intrusive presence of cables or molding in public hallways, and in some cases, even dangling from the outsides of buildings. Even when such an installation is already in place, regulations generally leave the cable installer, typically the local cable franchise, in monopoly possession of rights to use the local area network. Although this situation may change in the future, in parallel with current trends in electrical distribution and phone networks, at present landlords or cooperative tenants face serious legal difficulties attempting to distribute alternative source signals over proprietary cables. Circumventing this problem by installing a second independent cable distribution network in the same structure is clearly a less than appealing solution.
Hence there exists a clear need for alternative means of distributing video signals within multiple dwelling unit structures. An alternative distribution means is required to provide each unit or apartment with access to a large number of channels, typically between 50 and 100, with acceptable signal to noise ratio, and freedom from interference both to and from outside sources. The two general possibilities which present themselves, if a specialized network is not to be installed, are low-power broadcasting and use of an existing network of conducting paths. The former is generally ruled out because of interference and FCC regulation, although low-power broadcast devices are known to have been have been marketed for related purposes in direct contravention of regulation. Pondering the alternative of using pre-installed wiring, the technician will discover conductive paths in almost all existing structures, having terminations in every room or at least every unit, potentially including plumbing, power distribution wiring, and telephone wiring. No attempts are known to the inventor to use plumbing as a transmission medium, which plan would clearly present formidable difficulties. Telephone and power systems have not been designed for video signal transmission, but at least have been designed for providing electrically conductive paths. Appliances, however, are known to inject broadband RF energy into power lines, defeating efforts to utilize empty bandwidth above 50 Hz or 60 Hz alternating current. The present invention utilizes telephone lines as a transmission medium.
Telephone wires entail their own technical and regulatory problems as a medium for the transmission of video signals. As signal media, telephone lines represent an obsolescent system not designed to efficiently transmit RF (radio frequency) information, which for the purposes of the present invention essentially means all signals with frequency components above an audio baseband range, i.e. above approximately 4 kHz.
Analog phone lines concentrate most voice information below 4 kHz. Clearly, this band must be left unmolested to avoid interference with voice communications. Regulation imposes more stringent spectral requirements, however, limiting the amount of RF energy below 6 MHZ that may be injected into the public telephone network. Therefore the most conservative systems contemplating the injection of auxiliary carrier signals into phone wiring will not trespass on this band. Further problems arise from the fact that telephone wiring was not designed for radio frequency transmission. Telephone wiring lacks grounded shielding, which gives rise to further technical and legal problems. In particular, an unshielded conductor functions as an antenna. On the transmission side, this means the wires carrying RF signals radiate significant electromagnetic energy, resulting in signal attenuation. Boosting the signal will increase the maximum useful length of RF signal transmission on the wire, but simultaneously increase broadcast power and possibly cause interference to other devices, or run afoul of FCC radiation limits. Conversely, on the reception side, lack of shielding makes the system vulnerable to outside sources of interference. Maximum usable bandwidth similarly presents a trade-off between increasing utilization of the RF spectrum, and the increased radiation and attenuation at higher frequencies; signal loss through radiation being a monotonically increasing function of frequency. In general the problem of non-design utilization of unused transmission capacity in local area phone networks is one of choosing a power transmission spectrum extremizing an objective function weighted by both total information throughput and cost, and subject to technical and regulatory constraints on unintended interference with other devices. It is known as a practical matter that the solution to this optimization problem allows transmission of a small number of video channels, typically 2 or 3, across a single active telephone wiring pair, a number far lower than the number of program sources a consumer expects to be able to receive on demand. This design problem too must be overcome.
In the future, new structures will undoubtably be built with efficient broadband local area networks in place. Information distribution will be seen as a routine utility function, much as water, gas, sewer, power and phone hook ups are viewed today. For the near future, however, there will remain a strong demand to squeeze or piggy-back extra signals onto existing wiring, in cases where the economics can justify this approach over installing new local area networks. The problems to be overcome in this approach are outlined above, and the solution must thread between the twin hazards of unintended RF transmission and unacceptable signal degradation, at acceptable cost.
Extensive investigation into these problems is disclosed by Goodman et al. (U.S. Pat. No. 5,010,399), in which other prior art is also reviewed. However, the system disclosed by Goodman is adapted for installation in a single extended dwelling unit, such as a single family home, wherein a small number of single channel video sources and TV receivers are distributed. It is not obvious how to extend or adapt this household system to a multiple unit dwelling, where typically a single broadband source is to be simultaneously made available in its entirety at many remote locations.